To which I replied:
> Photoshop used to do it like this:
> Luminosity = 0.299 * R + 0.587 * G + 0.114 * B
>
> but I don't know if this method has been replaced by one that uses profiles.
> I could find out, if you really want to know.

I looked into it further today with PS 6.0.1 and it appears as though the
above equation (which are the NTSC weightings, BTW) is still used when PS
calculates the "Luminosity" histogram of an RGB image. I don't think
profiles or RGB working spaces have anything to do with it at all.

I haven't a clue how the Luminosity histogram is calculated for a CMYK
image.
--
Bruce J. Lindbloom, Pictographics Intl. Corp.